This invention relates to beamformers for use with an array of sonar transducers or electromagnetic radiating elements and, more particularly, to a beamformer operating at a reduced sampling rate.
Beamformers are used with arrays of sonar transducers for transmitting and receiving beams of sonic radiation. Similarly, beamformers are also used with arrays of electromagnetic radiating elements for transmitting and receiving beams of electromagnetic radiation. In the case of the forming of a receiving beam of radiation, the beamformer introduces temporal delays or phase shifts between the signals received from respective ones of the transducers or radiating elements in accordance with the differing times of arrival of a wavefront of radiation at the respective ones of the transducers or radiating elements in the array. Typically, phase shifters have been employed in radar systems utilizing an array antenna since the required sampling rates for the use of digital delay lines would be in excess of the capabilities of present day electronic circuits because of the relatively high carrier frequencies employed in most radar systems. In the case of sonar systems, wherein the carrier frequency of the radiation is substantially lower than that of radar systems, digital delay lines are frequently employed with the signals at the respective transducers being sampled at rates which are many times higher than the highest frequency of the sonic radiation to minimize the effects of temporal quantization in the beamforming process. By way of example, it has been found that the sampling of transducer signals at intersample intervals which are less than approximately one-tenth of a period of the radiation permits the formation of a receiving beam with substantially the same accuracy as can be provided at higher sampling rates. In a typical sonar operating at a sound frequency of ten kilohertz (kHz), the sampling rate of the signals of individual ones of the transducers would be at a rate of approximately 100 kHz. A sonar system employing the sampling of transducer signals and utilizing the delay lines for delaying the signal samples to produce a beam of radiation is disclosed in the U.S. Pat. No. 4,107,685 which issued in the name of Walter J. Martin et al on Aug. 15, 1978. While the use of digital sampling by analog-to-digital converters is disclosed in the aforementioned Martin patent, it is to be understood that sampling by means of sample-and-hold circuits followed by registers of charged-coupled devices (CCD)'s serving as the delay lines may also be employed.
A problem arises in that a high sampling rate necessitates the storage of many samples of the transducer signals, or signals of the radiating elements in the case of an electromagnetic system. Furthermore, the number of samples to be stored increases with the number of transducers in the array. And, as can be seen in the case of electromagnetic systems, the required sampling rate is so high as to preclude the use of digital delay lines in the systems operating at carrier frequencies above approximately 100 megahertz (MHz) with present technology. In systems of limited signal bandwidth, such as a sonar signal or radar signal having a bandwidth less than approximately ten percent of the carrier frequency, beamforming can be accomplished at the carrier or at intermediate frequencies (IF) by means of phase shifters. The beamforming operation by means of delay lines is applicable to both narrow and wide band signals. However, the beamforming operation by means of delay lines cannot be directly accomplished at IF because, during the translation of the carrier frequency to a lower frequency, there has been an alteration in the relationship between the period of the signal and the differences in the times of arrival of the wavefront upon the respective transducers of the array.
Since the invention is equally applicable to a beamformer operating with an array of sonar transducer elements and a beamformer operating with an array of electromagnetic radiating elements, the ensuing description of the invention is facilitated with reference to a beamforming operation utilizing only sonar transducers. However, it is to be understood that the terminology of transducer is to include the electromagnetic radiating element when the beamformer is to be incorporated in an electromagnetic system.